What material the heatsink is?

Yes you most certainly can!

No need for speculation or regurgitation of found wisdom, I’ve done it and it’s dropped the CPU and GPU temps by between 10-20c.

Follow the above guide on how to clean old thermal compound, but DO NOT apply the liquid metal in the same way thermal paste is applied - less is required and a different technique is used.

I used Conductonaut in 1g pack which offers the same value as any regular premium thermal paste, given how little is required (a pin-head sized blob). Here’s what I did.

1. Remove the logic board.
2. Remove the heat-pipe assembly.
3. Clean the old thermal compound from the CPU, GPU and heat-pipe assembly. Chances are it will be very dry, coming away easily. Use an old toothbrush to clean any stray compound from between surface mount devices on both components (DO NOT use anything firmer than a toothbrush to avoid damaging surface mount components). Ensure the die surfaces have a mirror-finish and the surrounding substrate is free of old compound. Don’t forget to clear any dust from the radiator fins on the heat-pipe.
4. Apply clear nail varnish to seal the surface mount components adjacent to the die on both CPU and GPU, this will protect against short-circuits and make it easier in the event you’re a little careless when applying the liquid metal. If the GPU has a protective plastic layer remove it before applying varnish.
5. Apply varnish to the alloy areas adjacent to the copper inserts on the heat-pipe. No need to cover the whole area, just enough so any overspill of liquid metal on the copper insert will not make contact with the alloy frame below. You don’t strictly need to do this as aluminium by nature forms a protective oxide layer which will resist the corrosive effects of gallium, but we all like to do a job right, first time, don’t we?
6. Wait until the varnish has dried, it won’t take more than a few minutes in a relatively warm room.
7. Following the supplied directions carefully apply Conductonaut (or your preferred brand of liquid metal) to both the die surfaces on the CPU and GPU, and the copper inserts on the heat-pipe assembly. The LM will apply easily to the die surfaces if they’ve been properly cleaned. The LM will adhere to the heat-pipe copper inserts when properly cleaned - if it doesn’t stick it’s not clean enough.
8. Refit the heat-pipe assembly to the logic board and reassemble the MacBook

The pre-LM temps on my MacBook weren’t great and continually high temps will shorten the life of discrete GPUs (a known problem on 27” iMacs, these can die due to being slowly cooked and ultimately requiring a GPU chip replacement). They’re now significantly lower with the CPU running at around 15c over ambient at idle, and the GPU at around 10c over ambient.

Some may speculate that a similar result could be had from conventional paste but in direct-die applications the benefits are more pronounced. Additionally, LM shouldn’t cure or dry out, and a single application should be all that’s required for the life of the device.

As for perceived value, such a tiny amount of LM is required for full die coverage that it doesn’t feel expensive given how much is left over to treat other devices.